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S. Ravikumar, A. Bradley, L.N. Thibos, X. Cheng; The Visual Impact of Phase and Amplitude Changes in Optically Aberrated Images . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2012.
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Purpose: To determine the visual impact of the phase and amplitude changes (taken together, or separately) that are induced by monochromatic optical aberrations of the eye. Methods: Subjects viewed isolated letters and 3x3 clusters of letters blurred computationally with Fourier optics (Cheng, et al. (2004), JOV 4:310) that simulated the effects of defocus, astigmatism, coma, and spherical aberration (SA) in various combinations. Aberrated letters were pre–compensated for the diffraction–limited optics (2.5 mm pupil) of the subject’s eye and then rendered with a high intensity digital projector to produce high–luminance (246 cd/m^2), monochromatic (556 nm) stimuli. The effect of aberrations on visual acuity was quantified with psychometric functions fit with Weibull templates. In addition to standard optical degradation in which both phase and amplitude changes coexist, Fourier optics allowed us to render targets selectively degraded either by the phase changes alone or by the amplitude demodulation alone. The impact of frequencies beyond the first MTF zero was also evaluated by selectively demodulating these frequencies. Results:When defocus or astigmatism coexists with SA of the same sign, the pi phase shifted portions of the spectrum have low amplitude and little visual impact. The converse is true when the SA has the opposite sign to that of the defocus or astigmatism. In these situations, phase changes can create diplopia and striking alterations in the apparent structure of the letters. Correcting these phase changes improves the appearance of the letters and correspondingly improves visual acuity (as little as about 50% of the VA loss is attributable to the MTF demodulations in these situations). Frequencies above the first MTF zero can be useful for character recognition. With coma, most of the reduction in VA is due to the MTF demodulation and not the phase changes. Conclusions: Although modulation–based metrics of image quality (e.g. area under the MTF) may be effective at capturing the visual impact of some aberrations, there are aberration combinations for which image quality is highly dependent upon optically–induced phase changes. The support NEI grant was EY05109 for this study.
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